At present high-molecular polyacetals are produced in the form of homopolymers and copolymers. Homopolymer-structure polyacetals are formed by polymerization of formaldehyde or cyclic oligomers thereof. Copolymers are obtained by copolymerization of formaldehyde with various comonomers taken in an amount of from 2 to 3% by weight, i.e. the starting feedstock for the production of high-molecular polyacetals is monomeric formaldehyde or cyclic oligomers of formaldehyde such as trioxane or tetraoxane (cf. N. S. Enikolopyan, S. A. Volfson "Chemistry and Processing of Polyformaldehyde," "Khimiya" Publishers, Moscow, 1968, in Russian).
One of the ways for improving the process for the production of high-molecular polyacetals is to reduce the number of the process stages. More promising in this respect is the way, when as the starting feedstock for the production of high-molecular polyacetals, use is made of monomeric formaldehyde, since this enables the elimination of the stages of synthesis of intermediate products.
Commercial production of formaldehyde is based on catalytic oxidative dehydrogenation of methanol in the presence of air oxygen on metallic or oxide-type catalysts. The resulting formaldehyde is in the form of a mixture with different components: H.sub.2 O, CH.sub.3 OH, HCOOH, N.sub.2, CO.sub.2, CO, H.sub.2, CH.sub.4, O.sub.2, and the like. The proportion of formaldehyde in the mixture depends on the catalysts employed and can vary from 2 to 19%. Recovery of formaldehyde from this mixture is effected, as a rule, by absorption thereof with water or alcohols. The resulting solutions can be used as a source for the production of concentrated monomeric formaldehyde or low-molecular polymers. Low-molecular polymers, in turn, can be used as a source for the production of monomeric formaldehyde through pyrolysis. To obtain concentrated monomeric formaldehyde, said solutions of formaldehyde are subjected to evaporation at a temperature within the range of from 110.degree. to 130.degree. C., followed by fractional condensation of vapours of water or an alcohol and separation thereof. Formaldehyde is thus produced in its monomeric form with the content of the main product ranging from 95 to 99%. The thus-recovered formaldehyde is subjected to further purification to the content of the main product of about 99.9%. For purification use is made of various physico-chemical methods based on profound condensation of impurities or chemical conversion thereof into different modifications, followed by separation.
Recovery and purification of monomeric concentrated formaldehyde constitute a multi-staged and complicated process due to an exclusively high chemical activity of formaldehyde and impossibility of storing it in its pure form (cf. J. Walker "Formaldehyde," Chemical Literature Publishing House, Moscow, 1957).
Therefore, difficulties associated with the recovery and purification of monomeric formaldehyde, impossibility of its storage in the pure form are serious obstacles in the way towards simplification of the process and reducing the production costs of the production of high-molecular polyacetals.
This is the basic factor hindering the extension of the manufacture of polyacetals on the basis of monomeric formaldehyde.
Known in the art are a number of processes for the production of high-molecular polyacetals by way of polymerization (copolymerization) of monomeric formaldehyde. Thus, known is a process comprising polymerization (copolymerization) of monomeric gaseous formaldehyde. To this end, the recovered and purified gaseous formaldehyde is fed into a reactor containing an inert hydrocarbon solvent and an ionic-type catalyst. In the case of copolymerization a comonomer such as cyclic formals, cyclic oxides is additionally introduced into the reactor. The polymerization (copolymerization) is carried out at a temperature within the range of from -50.degree. to +120.degree. C. The evolving reaction heat in the amount of 15.0-16.0 kcal/mol CH.sub.2 O is removed through the reactor walls by means of a suitable coolant (J. Furukawa and T. Saegusha "Polymerization of Aldehydes and Oxides," "Mir" Publishers, Moscow, 1965).
As it has been mentioned hereinabove, a great quantity of heat is evolved during polymerization (copolymerization) of gaseous formaldehyde.
It should be noted that the reaction heat can be removed by heat-transfer process only in the case of small-size reactors. In large-size reactors the ratio of the heat-transfer surface to the reactor volume is decreased, thus causing intentional decreasing of the polymerization (copolymerization) rate by lowering the unit load of formaldehyde in g CH.sub.2 O per liter of the liquid phase per unit of time.
It has been found that, all other factors being equal, molecular weight of the polymer decreases with a reduced unit load of formaldehyde. Thus, the polymer viscosity determined in a 0.5% solution of dimethylformamide at the temperature of 150.degree. C. diminishes from 0.68 to 0.57 dl/g respectively with the unit load being lowered from 15 to 2 g/l per minute.
The technique of the heat removal through the reactor walls does not make it possible to carry out a continuous process of polymerization (copolymerization) of gaseous formaldehyde due to deposition of the polymer film on the reactor walls, which substantially impairs the heat transfer process. The reactor should be periodically cleaned to remove the polymer film from the wall surface.
Technologically more efficient is the removal of the reaction heat by way of evaporation of the liquid phase and removal of its vapours from the reaction zone by means of an inert gas specifically introduced for this purpose.
Known in the art is a process for producing high-molecular polyacetals by polymerization of monomeric gaseous formaldehyde in an inert liquid containing an ionic catalyst (cf. Japanese Patent No. 8223 Cl. C 08 g 26, published on March 3, 1972). The process comprises supplying gaseous formaldehyde containing 99.8% of the main product into a reactor filled with n-hexane incorporating a catalyst--1.38% by weight of di-n-butyltindialurate at the temperature equal to 50.degree. C. At the same time, through a special pipe, separately from admission of formaldehyde, an inert gas, i.e. nitrogen, is fed. The resulting vapours of n-hexane are withdrawn from the reactor together with the inert gas. In this manner the reaction heat is removed and the required temperature conditions are maintained.
The above-discussed process makes it possible to solve the problem of removing the reaction heat; however, as the starting stock in this process use is made of concentrated formaldehyde and, hence, this process involves all the above-discussed difficulties, associated with the recovery and purification of gaseous formaldehyde.
In this respect, of certain interest is the process for producing high-molecular polyacetals, wherein as the starting feedstock use is made of a solution of formaldehyde in an inert hydrocarbon, formaldehyde being obtained from the reaction gases of catalytic oxidative dehydrogenation of methanol (cf. French Pat. No. 1,336,994). According to this Patent, the reaction gases from oxidation of methanol are purified from H.sub.2 O, CH.sub.3 OH and HCOOH contaminants by fractional condensation and removal of these impurities. Thereafter, to carry out profound purification to the content of the contaminants down to 0.1% by weight, the remaining gaseous mixture is passed through a vessel containing n-heptane incorporating 0.01% of n-butylamine at the temperature of -10.degree. C.
The reaction mixture decontaminated from H.sub.2 O, CH.sub.3 OH and HCOOH is then fed into an absorber to recover pure formaldehyde by absorption thereof with an inert hydrocarbon (toluene) at the temperature of -70.degree. C. The thus-obtained solution of formaldehyde in toluene is free from admixtures of other components otherwise present in the starting reaction gases available from oxidation of methanol, namely: N.sub.2, CO.sub.2, CO, CH.sub.4, H.sub.2, O.sub.2, and the like, and constitutes the starting feedstock for the production of high-molecular polyacetals. This solution of pure formaldehyde is admitted into the reactor, whereinto added is a polymerization catalyst, i.e. n-butylamine. Polymerization of formaldehyde takes place in the reactor.
This process is distinguished from the prior art processes described hereinbefore by that the solution of formaldehyde in an inert liquid as used at the polymerization stage is obtained directly from the reaction gases of the oxidative dehydrogenation of methanol after decontaminating them from H.sub.2 O, CH.sub.3 OH and HCOOH. However, the necessity in obtaining the solution of formaldehyde in an inert liquid substantially complicates the process and lowers its economical efficiency.